1. Field of the Invention
The invention lies in the field of systems of hollow guiding members for cable-like elements, such as cable ducts. More in particular, it concerns a method for removing and/or detecting moisture penetrating into a hollow guiding member of a system of hollow guiding members. It further concerns a system of hollow guiding members for cable-like elements for applying the method according to the invention. Finally, it concerns coupling members for application in a system of hollow guiding members according to the invention.
2. Prior Art
It is customary not to bury cables, particularly glass-fibre cables, directly in the ground, but first to lay a tubular protection, hereinafter referred to as cable duct or duct for short, in the ground and subsequently to install the glass-fibre cable in this laid duct. Such cable duct has two major functions, namely, as guiding member for guiding a cable during installation in the desired direction, and as protection member for protecting a cable once installed in the duct against damaging outside influences. Digging trenches or ditches in the ground to subsequently lay a cable duct therein, however, is an expensive affair. Preferably, therefore, cable ducts are applied which, once buried, continue to meet future needs of new, additional or substitute cables for as long as possible. These may be cable ducts consisting of a singular tube, from which a cable once installed therein can be simply removed and replaced by a new cable. Furthermore, a cable duct with a relatively large diameter, such as, e.g., is disclosed in reference [1], may contain several cable ducts with smaller diameters, referred to as sub-ducts. In this connection, there is introduced into a large, primary tubular duct already buried a bundle of smaller, secondary tubular ducts, referred to as sub-ducts. Subsequently, at any desired point in time cables may be installed in the sub-ducts as the need arises. Here, such sub-ducts not only offer an additional protection to the cables, but also simplify the routing of the cables. Moreover, such sub-ducts make it possible to selectively remove cables and to replace them by new ones. This technique is not only applicable to complete cables, either electric cables or optical-fibre cables, provided with a traction element and various protective sheaths against outside influences. As is disclosed in reference [2], this technique is applicable down to core level, with the cable to be introduced into a sub-duct comprising only one or a few cores, such as copper wires or glass fibres. Reference [3] discloses a technique which, inter alia, applies a bundle of sub-ducts consisting, as it were, of a cable-sheath body provided with several tubular cavities wherein fibre elements may be installed as the need arises with the help of a blast-air technique.
A singular cable duct, but definitely a combination of a primary and a secondary duct, for each cable installed therein in fact forms a (double) wall around the cable, which may offer a (double) protection against negative outside influences, such as against penetrating moisture. Therefore, cables to be installed might be of a simpler construction if said (double) wall around the cable meets certain conditions. Thus, a cable need not be provided with a (metal) moisture screen if no moisture from outside the primary duct is able to penetrate into a secondary duct, as a result of which the cable may be implemented more lightly and cheaply, and may be smaller in size. The current frequently used cable ducts, wherein complete optical-fibre cables are installed, often are tubes made of synthetic material, such as HDPE. Due to diffusion, however, such synthetic materials are permeable for water. This means that, even if in such a tube there is no leak, water may still gradually accumulate within such tube, which may subsequently affect a cable not protected against it. From manufacturing and cost considerations it would be a great advantage to be able to use such synthetic tubes, apart from the inner and outer diameters, without modification, both for the primary and for the secondary duct, particularly also for a cable without moisture screen.
In addition it is disclosed, such as, e.g., in references [4] and [5], particularly in the event of optical-fibre cables, to pressurise hollow spaces between the optical fibres lying within the sheath of the cable over a length of the cable with compressed gas in order to prevent moisture penetration through the cable sheath. This functions well in the event of leaks of not-too-great dimensions. Moisture penetrating due to diffusion, however, cannot be stopped in this manner, since for diffusion the driving force does not lie in the total pressure prevailing in the hollow spaces, but rather in the partial pressure of the water vapour present therein.
A variant of this technique is still disclosed in reference [6], in which an Al tubelet, provided with a glass fibre, of a composite overhead glass-fibre cable, through an open end in a gas-tight interconnection box, is first blown through with compressed gas (dry nitrogen). Subsequently, the other end is closed off gas-tight. The nitrogen gas is retained under pressure in the interconnection box and in the tubelet, as a result of which the formation of frozen water drops in the event of a decline in ambient temperature is prevented. Furthermore there is disclosed, as in reference [7], the forcing of dry nitrogen gas under high pressure through the free space between the electric cores within the sheath of an underground cable from one cable end to the other. As a result, the cable innards are kept dry to prevent the formation of electrochemical trees in the synthetic insulation of the cable sheath.